This invention relates to a unique head structure for a digital magnetic recording assembly.
The basic head assembly for a digital magnetic recording device, such as a reel-to-reel tape drive, consists of an approximately U-shaped core made from some high permeability material, with a small non-magnetic gap at the top and an electric coil wound around the base of the core. The tape or other magnetic media moves across the gap at a predetermined distance, usually on the order of 10.sup.-4 -10.sup.-5 inches.
In the write operation, a time-varying electric current is sent through the coil to produce a time-varying magnetic field in the core. The magnetic flux lines bridge the non-magnetic gap and create a fringe field above it, which in turn lays down a spatially varying magnetic pattern on the tape. In the read operation, the magnetic pattern on the tape induces a time-varying magnetic field in the core and an associated current in the coil.
Thus, successful recording depends on precise control of the contact between the tape and the fringe field. Both the tape-head distance and the tangential pressure of the tape on the head must be maintained within very narrow tolerances at the gap. if the distance there is too great and the pressure too low, the tape may not record. On the other hand, too small a distance or too great a pressure may cause physical damage to the tape or head, or the recording of phantom signals. Even small variations can lead to erratic recording performance.
In the prior art, wide island heads have been widely used. In these heads, the core is embedded in, and flush with the surfaces of, hard, highly polished sliders made from materials such as chrome. The rapidly moving tape carries air with it, and the pressure of the air trapped between the tape and the smoothly curved surfaces of the sliders creates an aerodynamic cushion for the tape, whose height or thickness is proportional to the speed of the tape. The tape travels over the sliders for a considerable distance on either side of the head, as much as 5.times.10.sup.-2 inches in all, during which this controlled aerodynamic separation is maintained. In the past, it was believed that the smooth, continuously curved slider of the wide island head was required to avoid turbulence at the gap and maintain the cushion reliably within the required tolerances.
Wide island heads do provide a reliable aerodynamic cushion. The cushion alone, however, does not ensure good contact between the tape and the magnetic field for the life of the recording assembly. Relatively high pressures are required to provide contact on the wide, flat surface of the head, so the entire head wears quickly, and the soft core wears much more quickly than the sliders. As the core wears down, the distance between the tape and core increases so that the signals lose resolution and the noise on the tape increases. This leads to signal losses on the recording tracks and, even worse, synchronization failures on the timing track which can make the tape unusable. Eventually, and in commercial tape assemblies often as frequently as every six months, the sliders must be ground down, a complex and expensive procedure which involves taking apart, reassembling and recalibrating the entire tape drive. Since cores cannot be replaced onsite, the tape drive must be "down" for several weeks per year while the cores are being replaced.
Even before significant wear takes place, wide island heads are particularly susceptible to errors caused by non-uniformities in the tape and contaminants. Non-conductive contaminants merely blanket the signal over the area of contamination but conductive contaminants and high-spots in the tape can cause a false signal over the entire width of the head. Thus, the error rate on the tape increases exponentially with the width of the head.